The histochemical regional ischemic changes have been directly related to the duration of the blood supply restriction to the brain and the following period of reestablished circulation. At the end of ischemic insult and after 1 hour of common carotid artery clip release, no differences in enzymatic activities were seen between ischemic and non-ischemic Ammon's horn. Twenty hours after restoration of cerebral circulation, the oxidative mitochondrial enzymes S-DH, L-DH, G-DH, G-6-DH and C-OH were markedly increased in the pyramidal cells of sabiculum and H3 sector of the hippocampus. The proteolytic enzyme aminopeptidase and the Golgi's marker TPP-ase were also increased in the same part of Ammon's horn.